Profiling the Binding Sites of RNA-Binding Proteins with Nucleotide Resolution Using iCLIP

Long-read transcriptome sequencing of CLL and MDS patients uncovers molecular effects of SF3B1 mutations

Mutations in splicing factor 3B subunit 1 (SF3B1) frequently occur in patients with chronic lymphocytic leukemia (CLL) and myelodysplastic syndromes (MDSs). These mutations have different effects on the disease prognosis with beneficial effect in MDS and worse prognosis in CLL patients. A full-length transcriptome approach can expand our knowledge on SF3B1 mutation effects on RNA splicing and its contribution to patient survival and treatment options. We applied long-read transcriptome sequencing (LRTS) to 44 MDS and CLL patients, as well as two pairs of isogenic cell lines with and without SF3B1 mutations, and found >60% of novel isoforms. Splicing alterations were largely shared between cancer types and specifically affected the usage of introns and 3' splice sites. Our data highlighted a constrained window at canonical 3' splice sites in which dynamic splice-site switches occurred in SF3B1-mutated patients. Using transcriptome-wide RNA-binding maps and molecular dynamics simulations, we showed multimodal SF3B1 binding at 3' splice sites and predicted reduced RNA binding at the second binding pocket of SF3B1K700E Our work presents the hitherto most-complete LRTS study of the SF3B1 mutation in CLL and MDS and provides a resource to study aberrant splicing in cancer. Moreover, we showed that different disease prognosises result most likely from the different cell types expanded during carcinogenesis rather than different mechanisms of action of the mutated SF3B1. These results have important implications for understanding the role of SF3B1 mutations in hematological malignancies and other related diseases.

Mapping protein-RNA binding in plants with individual-nucleotide-resolution UV cross-linking and immunoprecipitation (plant iCLIP2)

Despite crucial roles of RNA-binding proteins (RBPs) in plant physiology and development, methods for determining their transcriptome-wide binding landscape are less developed than those used in other model organisms. Cross-linking and immunoprecipitation (CLIP) methods (based on UV-mediated generation of covalent bonds between RNAs and cognate RBPs in vivo, purification of the cross-linked complexes and identification of the co-purified RNAs by high-throughput sequencing) have been applied mainly in mammalian cells growing in monolayers or in translucent tissue. We have developed plant iCLIP2, an efficient protocol for performing individual-nucleotide-resolution CLIP (iCLIP) in plants, tailored to overcome the experimental hurdles posed by plant tissue. We optimized the UV dosage to efficiently cross-link RNA and proteins in plants and expressed epitope-tagged RBPs under the control of their native promoters in loss-of-function mutants. We select epitopes for which nanobodies are available, allowing stringent conditions for immunopurification of the RNA-protein complexes to be established. To overcome the inherently high RNase content of plant cells, RNase inhibitors are added and the limited RNA fragmentation step is modified. We combine the optimized isolation of RBP-bound RNAs with iCLIP2, a streamlined protocol that greatly enhances the efficiency of library preparation for high-throughput sequencing. Plant researchers with experience in molecular biology and handling of RNA can complete this iCLIP2 protocol in ~5 d. Finally, we describe a bioinformatics workflow to determine targets of Arabidopsis RBPs from iCLIP data, covering all steps from downloading sequencing reads to identifying cross-linking events ( https://github.com/malewins/Plant-iCLIPseq ), and present the R/Bioconductor package BindingSiteFinder to extract reproducible binding sites ( https://bioconductor.org/packages/release/bioc/html/BindingSiteFinder.html ).

The landscape of alternative polyadenylation during EMT and its regulation by the RNA-binding protein Quaking

Epithelial-mesenchymal transition (EMT) plays important roles in tumour progression and is orchestrated by dynamic changes in gene expression. While it is well established that post-transcriptional regulation plays a significant role in EMT, the extent of alternative polyadenylation (APA) during EMT has not yet been explored. Using 3' end anchored RNA sequencing, we mapped the alternative polyadenylation (APA) landscape following Transforming Growth Factor (TGF)-β-mediated induction of EMT in human mammary epithelial cells and found APA generally causes 3'UTR lengthening during this cell state transition. Investigation of potential mediators of APA indicated the RNA-binding protein Quaking (QKI), a splicing factor induced during EMT, regulates a subset of events including the length of its own transcript. Analysis of QKI crosslinked immunoprecipitation (CLIP)-sequencing data identified the binding of QKI within 3' untranslated regions (UTRs) was enriched near cleavage and polyadenylation sites. Following QKI knockdown, APA of many transcripts is altered to produce predominantly shorter 3'UTRs associated with reduced gene expression. These findings reveal the changes in APA that occur during EMT and identify a potential role for QKI in this process.

NOseq: amplicon sequencing evaluation method for RNA m6A sites after chemical deamination

Methods for the detection of m6A by RNA-Seq technologies are increasingly sought after. We here present NOseq, a method to detect m6A residues in defined amplicons by virtue of their resistance to chemical deamination, effected by nitrous acid. Partial deamination in NOseq affects all exocyclic amino groups present in nucleobases and thus also changes sequence information. The method uses a mapping algorithm specifically adapted to the sequence degeneration caused by deamination events. Thus, m6A sites with partial modification levels of ∼50% were detected in defined amplicons, and this threshold can be lowered to ∼10% by combination with m6A immunoprecipitation. NOseq faithfully detected known m6A sites in human rRNA, and the long non-coding RNA MALAT1, and positively validated several m6A candidate sites, drawn from miCLIP data with an m6A antibody, in the transcriptome of Drosophila melanogaster. Conceptually related to bisulfite sequencing, NOseq presents a novel amplicon-based sequencing approach for the validation of m6A sites in defined sequences.

The RNA-binding profile of the splicing factor SRSF6 in immortalized human pancreatic β-cells

In pancreatic β-cells, the expression of the splicing factor SRSF6 is regulated by GLIS3, a transcription factor encoded by a diabetes susceptibility gene. SRSF6 down-regulation promotes β-cell demise through splicing dysregulation of central genes for β-cells function and survival, but how RNAs are targeted by SRSF6 remains poorly understood. Here, we define the SRSF6 binding landscape in the human pancreatic β-cell line EndoC-βH1 by integrating individual-nucleotide resolution UV cross-linking and immunoprecipitation (iCLIP) under basal conditions with RNA sequencing after SRSF6 knockdown. We detect thousands of SRSF6 bindings sites in coding sequences. Motif analyses suggest that SRSF6 specifically recognizes a purine-rich consensus motif consisting of GAA triplets and that the number of contiguous GAA triplets correlates with increasing binding site strength. The SRSF6 positioning determines the splicing fate. In line with its role in β-cell function, we identify SRSF6 binding sites on regulated exons in several diabetes susceptibility genes. In a proof-of-principle, the splicing of the susceptibility gene LMO7 is modulated by antisense oligonucleotides. Our present study unveils the splicing regulatory landscape of SRSF6 in immortalized human pancreatic β-cells.

Ythdf is a N6-methyladenosine reader that modulates Fmr1 target mRNA selection and restricts axonal growth in Drosophila

N6‐methyladenosine (m⁶A) regulates a variety of physiological processes through modulation of RNA metabolism. This modification is particularly enriched in the nervous system of several species, and its dysregulation has been associated with neurodevelopmental defects and neural dysfunctions. In Drosophila, loss of m⁶A alters fly behavior, albeit the underlying molecular mechanism and the role of m⁶A during nervous system development have remained elusive. Here we find that impairment of the m⁶A pathway leads to axonal overgrowth and misguidance at larval neuromuscular junctions as well as in the adult mushroom bodies. We identify Ythdf as the main m⁶A reader in the nervous system, being required to limit axonal growth. Mechanistically, we show that the m⁶A reader Ythdf directly interacts with Fmr1, the fly homolog of Fragile X mental retardation RNA binding protein (FMRP), to inhibit the translation of key transcripts involved in axonal growth regulation. Altogether, this study demonstrates that the m⁶A pathway controls development of the nervous system and modulates Fmr1 target transcript selection.

The cytoplasmic SYNCRIP mRNA interactome of mammalian neurons

SYNCRIP, a member of the cellular heterogeneous nuclear ribonucleoprotein (hnRNP) family of RNA binding proteins, regulates various aspects of neuronal development and plasticity. Although SYNCRIP has been identified as a component of cytoplasmic RNA granules in dendrites of mammalian neurons, only little is known about the specific SYNCRIP target mRNAs that mediate its effect on neuronal morphogenesis and function. Here, we present a comprehensive characterization of the cytoplasmic SYNCRIP mRNA interactome using iCLIP in primary rat cortical neurons. We identify hundreds of bona fide SYNCRIP target mRNAs, many of which encode for proteins involved in neurogenesis, neuronal migration and neurite outgrowth. From our analysis, the stabilization of mRNAs encoding for components of the microtubule network, such as doublecortin (Dcx), emerges as a novel mechanism of SYNCRIP function in addition to the previously reported control of actin dynamics. Furthermore, we found that SYNCRIP synergizes with pro-neural miRNAs, such as miR-9. Thus, SYNCRIP appears to promote early neuronal differentiation by a two-tier mechanism involving the stabilization of pro-neural mRNAs by direct 3'UTR interaction and the repression of anti-neural mRNAs in a complex with neuronal miRISC. Together, our findings provide a rationale for future studies investigating the function of SYNCRIP in mammalian brain development and disease.

Makorin 1 controls embryonic patterning by alleviating Bruno1-mediated repression of oskar translation

Makorins are evolutionary conserved proteins that contain C₃H-type zinc finger modules and a RING E3 ubiquitin ligase domain. In Drosophila, maternal Makorin 1 (Mkrn1) has been linked to embryonic patterning but the mechanism remained unsolved. Here, we show that Mkrn1 is essential for axis specification and pole plasm assembly by translational activation of oskar (osk). We demonstrate that Mkrn1 interacts with poly(A) binding protein (pAbp) and binds specifically to osk 3’ UTR in a region adjacent to A-rich sequences. Using Drosophila S2R+ cultured cells we show that this binding site overlaps with a Bruno1 (Bru1) responsive element (BREs) that regulates osk translation. We observe increased association of the translational repressor Bru1 with osk mRNA upon depletion of Mkrn1, indicating that both proteins compete for osk binding. Consistently, reducing Bru1 dosage partially rescues viability and Osk protein level in ovaries from Mkrn1 females. We conclude that Mkrn1 controls embryonic patterning and germ cell formation by specifically activating osk translation, most likely by competing with Bru1 to bind to osk 3’ UTR.

To ensure accurate development of the Drosophila embryo, proteins and mRNAs are positioned at specific sites within the embryo. Many of these factors are produced and localized during the development of the egg in the mother. One protein essential for this process that has been heavily studied is Oskar (Osk), which is positioned at the posterior pole. During the localization of osk mRNA, its translation is repressed by the RNA-binding protein Bruno1 (Bru1), ensuring that Osk protein is not present outside of the posterior where it is harmful. At the posterior pole, osk mRNA is activated through mechanisms that are not yet understood. In this work, we show that the conserved protein Makorin 1 (Mkrn1) is a novel factor involved in the translational activation of osk. Mkrn1 binds specifically to osk mRNA, overlapping with a binding site of Bru1, thus alleviating the association of Bru1 with osk. Moreover, Mkrn1 is stabilized by poly(A) binding protein (pAbp), a translational activator that binds osk mRNA in close proximity to one Mkrn1 binding site. Our work thus helps to answer a long-standing question in the field, providing insight about the function of Mkrn1 and more generally into embryonic patterning in animals.

The RNA-binding ubiquitin ligase MKRN1 functions in ribosome-associated quality control of poly(A) translation

BACKGROUND

Cells have evolved quality control mechanisms to ensure protein homeostasis by detecting and degrading aberrant mRNAs and proteins. A common source of aberrant mRNAs is premature polyadenylation, which can result in non-functional protein products. Translating ribosomes that encounter poly(A) sequences are terminally stalled, followed by ribosome recycling and decay of the truncated nascent polypeptide via ribosome-associated quality control.

RESULTS

Here, we demonstrate that the conserved RNA-binding E3 ubiquitin ligase Makorin Ring Finger Protein 1 (MKRN1) promotes ribosome stalling at poly(A) sequences during ribosome-associated quality control. We show that MKRN1 directly binds to the cytoplasmic poly(A)-binding protein (PABPC1) and associates with polysomes. MKRN1 is positioned upstream of poly(A) tails in mRNAs in a PABPC1-dependent manner. Ubiquitin remnant profiling and in vitro ubiquitylation assays uncover PABPC1 and ribosomal protein RPS10 as direct ubiquitylation substrates of MKRN1.

CONCLUSIONS

We propose that MKRN1 mediates the recognition of poly(A) tails to prevent the production of erroneous proteins from prematurely polyadenylated transcripts, thereby maintaining proteome integrity.

Decoding a cancer-relevant splicing decision in the RON proto-oncogene using high-throughput mutagenesis

Mutations causing aberrant splicing are frequently implicated in human diseases including cancer. Here, we establish a high-throughput screen of randomly mutated minigenes to decode the cis-regulatory landscape that determines alternative splicing of exon 11 in the proto-oncogene MST1R (RON). Mathematical modelling of splicing kinetics enables us to identify more than 1000 mutations affecting RON exon 11 skipping, which corresponds to the pathological isoform RON∆165. Importantly, the effects correlate with RON alternative splicing in cancer patients bearing the same mutations. Moreover, we highlight heterogeneous nuclear ribonucleoprotein H (HNRNPH) as a key regulator of RON splicing in healthy tissues and cancer. Using iCLIP and synergy analysis, we pinpoint the functionally most relevant HNRNPH binding sites and demonstrate how cooperative HNRNPH binding facilitates a splicing switch of RON exon 11. Our results thereby offer insights into splicing regulation and the impact of mutations on alternative splicing in cancer.

Individual Nucleotide Resolution UV Cross-Linking and Immunoprecipitation (iCLIP) to Determine Protein-RNA Interactions

RNA-binding proteins (RBPs) interact with and determine the fate of many cellular RNA transcripts. In doing so they help direct many essential roles in cellular physiology, while their perturbed activity can contribute to disease etiology. In this chapter we detail a functional genomics approach, termed individual nucleotide resolution UV cross-linking and immunoprecipitation (iCLIP), that can determine the interactions of RBPs with their RNA targets in high throughput and at nucleotide resolution. iCLIP achieves this by exploiting UV-induced covalent cross-links formed between RBPs and their target RNAs to both purify the RBP-RNA complexes under stringent conditions, and to cause reverse transcription stalling that then identifies the direct cross-link sites in the high throughput sequenced cDNA libraries.

miR-200/375 control epithelial plasticity-associated alternative splicing by repressing the RNA-binding protein Quaking

Members of the miR‐200 family are critical gatekeepers of the epithelial state, restraining expression of pro‐mesenchymal genes that drive epithelial–mesenchymal transition (EMT) and contribute to metastatic cancer progression. Here, we show that miR‐200c and another epithelial‐enriched miRNA, miR‐375, exert widespread control of alternative splicing in cancer cells by suppressing the RNA‐binding protein Quaking (QKI). During EMT, QKI‐5 directly binds to and regulates hundreds of alternative splicing targets and exerts pleiotropic effects, such as increasing cell migration and invasion and restraining tumour growth, without appreciably affecting mRNA levels. QKI‐5 is both necessary and sufficient to direct EMT‐associated alternative splicing changes, and this splicing signature is broadly conserved across many epithelial‐derived cancer types. Importantly, several actin cytoskeleton‐associated genes are directly targeted by both QKI and miR‐200c, revealing coordinated control of alternative splicing and mRNA abundance during EMT. These findings demonstrate the existence of a miR‐200/miR‐375/QKI axis that impacts cancer‐associated epithelial cell plasticity through widespread control of alternative splicing.

In vitro iCLIP-based modeling uncovers how the splicing factor U2AF2 relies on regulation by cofactors

Alternative splicing generates distinct mRNA isoforms and is crucial for proteome diversity in eukaryotes. The RNA-binding protein (RBP) U2AF2 is central to splicing decisions, as it recognizes 3′ splice sites and recruits the spliceosome. We establish “in vitro iCLIP” experiments, in which recombinant RBPs are incubated with long transcripts, to study how U2AF2 recognizes RNA sequences and how this is modulated by trans-acting RBPs. We measure U2AF2 affinities at hundreds of binding sites and compare in vitro and in vivo binding landscapes by mathematical modeling. We find that trans-acting RBPs extensively regulate U2AF2 binding in vivo, including enhanced recruitment to 3′ splice sites and clearance of introns. Using machine learning, we identify and experimentally validate novel trans-acting RBPs (including FUBP1, CELF6, and PCBP1) that modulate U2AF2 binding and affect splicing outcomes. Our study offers a blueprint for the high-throughput characterization of in vitro mRNP assembly and in vivo splicing regulation.

Experimental approaches to studying the nature and impact of splicing variation in zebrafish

From a fixed number of genes carried in all cells, organisms create considerable diversity in cellular phenotype through differential regulation of gene expression. One prevalent source of transcriptome diversity is alternative pre-mRNA splicing, which is manifested in many different forms. Zebrafish models of splicing dysfunction due to mutated spliceosome components provide opportunity to link biochemical analyses of spliceosome structure and function with whole organism phenotypic outcomes. Drawing from experience with two zebrafish mutants: cephalophŏnus (a prpf8 mutant, isolated for defects in granulopoiesis) and caliban (a rnpc3 mutant, isolated for defects in digestive organ development), we describe the use of glycerol gradient sedimentation and native gel electrophoresis to resolve components of aberrant splicing complexes. We also describe how RNAseq can be employed to examine relatively rare alternative splicing events including intron retention. Such experimental approaches in zebrafish can promote understanding of how splicing variation and dysfunction contribute to phenotypic diversity and disease pathogenesis.